The bonding or lamination of more sheets of paper or paperboard, either with each other or with sheets or films of non-cellulosic materials is a commonly utilized technique, especially in the preparation of materials for packaging, for obtaining composite sheets having the combined properties of the two or more component sheets, or for preparing sheets endowed with a high weight, not directly obtainable by papermaking machines.
According to the conventional techniques, such bonding is accomplished by applicating an adhesive on the surfaces of the sheets and then by laminating the superimposed sheets in a press or between rollers, in order to achieve the adhesion between the component sheets. Such technologies exhibit some limits due to the type of adhesive to be used in relation to the nature of the sheets and to the high temperatures required, which cause degradation of the cellulose or of the other types of fibres which may be present.
More recently, the availability on the market of thermoplastic synthetic fibres capable of fully or partially replacing the cellulose in the preparation of paper sheets or articles has enabled to prepare bonded or laminated articles of the type described hereinbefore starting from sheets containing more or less high percentages of such fibres. These sheets are superimposed, then heated and compressed at a temperature higher than the melting temperature of the synthetic fibres contained therein, which, by consequence, act as bonding agents between the various layers.
This method, which may be defined as thermowelding method, exhibits however the drawback of requiring too high operating temperatures, generally exceeding by 20.degree.-30.degree. C. the melting temperature of the polymer which forms the thermoplastic synthetic fibres contained in the sheets.
That happens because it is necessary, for purposes of adhesion, that the molten polymer should possess a certain mobility or flowability under the conditions in which it is operated.
Such high temperatures are generally prejudicial to the mechanical characteristics of the cellulosic fibres, if any, present in the sheets; furthermore said high temperatures do not permit to prevent a complete melting of the remaining thermoplastic fibres contained in the structure and not interested in the adhesion between the sheets, what results in the loss of their fibrous morphology.
Another drawback caused by the necessity of operating at such high temperature resides in the fact that, under such conditions, a certain worsening of the mechanical properties of the sheets occurs, especially in the flexural modulus, what proves to be particularly disadvantageous when the sheets must retain, at such temperatures, a preassigned shape, obtained for example by preceding drawing, embossing and the like.